JP2010215944A - Shot for peening, and method for producing the shot - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a shot for peening which simultaneously form a compressive residual stress layer and a coating layer of a highly corrosion resistant element, and can improve the corrosion resistance of the material in a high temperature-high pressure water environment, and to provide a method for producing the same. <P>SOLUTION: The shot for peening is made of an iron-chromium-nickel alloy, which is used for a surface modification method where a compressive residual stress layer is formed on the surface layer of a material, and chromium as an element having high corrosion resistance is applied to the surface of the material; wherein the content of chromium in the surface layer 2 of the shot to be used is higher than that in the central part 1. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention relates to a shot for shot peening for applying a residual compressive stress to the surface of a material by a shot peening method, and a method for manufacturing the shot.

  Austenitic stainless steel or nickel-base alloy that has excellent corrosion resistance in high-temperature and high-pressure water is used for the reactor internal structure. However, stress corrosion cracking (SCC) occurs in welded parts with residual tensile stress or machined parts. It is known that it can occur.

As countermeasures, countermeasures have been taken to prevent the occurrence of SCC by adopting a material having higher SCC resistance and controlling water quality or improving residual stress. In particular, a stress improvement process called peening has been developed and applied for the purpose of suppressing the occurrence of SCC by converting tensile residual stress, which is a factor of SCC, into compression (see Patent Document 1).

Moreover, the technique which forms the surface layer which has high corrosion resistance on the surface for the purpose of the reliability improvement of not only the nuclear field but the apparatus used in a corrosive environment, components, and lifetime extension is disclosed (refer patent document 2). Furthermore, a method for improving surface hardness or lubricity by applying a surface modification method by laser peening to parts that require sliding properties used in transportation equipment and rotating equipment such as automobiles has been released. (See Patent Document 3). In addition, a method for performing material improvement coating by ultrasonic peening has been disclosed (see Patent Document 4).

US Pat. No. 6,993,948 JP 2000-34581 A JP 2006-320907 A JP2007-197764A

  Boiling water reactors (BWR) and pressurized water reactors (PWR) are operating in Japan and abroad as commercial light water reactors, but all major equipment is exposed to high-temperature and high-pressure water environments. In particular, in an aging plant that has been in operation for a long time since the start of operation, there has been reported an example of damage caused by superposition of residual stress, environment, and material deterioration called stress corrosion cracking (SCC). Residual stress is mainly the tensile residual stress generated in the welds of structures. In the case of BWR, environmental factors include dissolved oxygen, various impurities, temperature in PWR, and hydrogen in a specific concentration range. . Moreover, it is said that the decrease in the chromium concentration in the vicinity of the grain boundary is partly due to the material.

As a countermeasure against SCC, various countermeasure methods for BWR and PWR have been developed and applied to actual machines. From the standpoint of suppressing the residual stress factor, the peening method has been applied to both BWR and PWR plants.

In the old days, shot peening (SP) has been applied as an SCC countermeasure for the steam generator (SG) of PWR, and its effect has been demonstrated. Laser peening (LP) and water jet peening (WJP), which are excellent in remote workability, have been put to practical use as a residual stress improvement technique for in-furnace equipment of BWR and PWR. Moreover, from the point of environmental factor suppression, the reduction of the dissolved oxygen concentration by the hydrogen injection in a BWR plant and the noble metal injection technique are applied, and the effect is being demonstrated. Furthermore, as a material improvement, the application of a nickel-base weld metal with higher chromium compared to conventional materials, or a high-chromium nickel-base alloy has been developed and applied to PWR replacement devices and the like.

However, each method improves only one of the three factors, and depending on the equipment, the influence of the machining layer inevitably formed by the manufacturing history, heat during operation, external load history, etc. overlap. There is also a concern that SCC is induced. Therefore, if a process that can improve multiple factors at the same time at low cost is developed, it will be possible to more reliably prevent the occurrence of SCC, improve the reliability of nuclear reactor equipment, and contribute to the realization of a highly economical plant. be able to.

  The present invention has been made to solve the above-described problems, and simultaneously forms a compressive residual stress layer and a coating layer of a high corrosion resistance element on the surface of the material, thereby improving the corrosion resistance of the material in a high temperature and high pressure water environment. A peening shot and a method for manufacturing the same are proposed.

In order to solve the above problems, in the present invention, an iron-chromium-nickel used in a surface modification method for forming a compressive residual stress layer on a material surface layer and imparting chromium, which is an element having high corrosion resistance, to the material surface. A shot for peening made of an iron-chromium-nickel alloy and a spherical iron-chromium-nickel alloy, characterized in that the amount of chromium in the surface layer of the shot used is higher than that of the center portion. The method for producing the peening shot is characterized in that a surface layer containing a large amount of chromium in the surface layer is formed by heat treatment in a vacuum.

By the peening shot according to the present invention and the manufacturing method of the shot, it is possible to simultaneously form a compressive residual stress layer and a coating layer of a high corrosion resistance element on the surface of the material, thereby improving the corrosion resistance of the material in a high temperature and high pressure water environment. it can.

The conceptual diagram of the shot for peening which concerns on this invention is shown, (a) is a longitudinal cross-sectional view, (b) is the A section enlarged view of (a), (c) is the distribution map of the chromium content of the surface vicinity. The flowchart which shows the flow of the shot for peening manufacture by this invention. The conceptual diagram of the principle in which the layer containing much chromium is formed on the shot surface is shown, (a) is a longitudinal sectional view, (b) is an enlarged view of C part of (a), (c) is the amount of chromium near the surface. Distribution map.

  Embodiments of the shot and surface modification method according to the present invention will be described below with reference to the drawings.

(Example)
A first embodiment according to the present invention will be described with reference to FIG. FIG. 1A is a diagram schematically showing a cross section of a peening shot according to the present invention. In addition,% of the component shown below has shown the mass%.

In FIG. 1, for example, a substantially spherical shape made of SUS310S having 25.28% chromium, 19.38% nickel, 0.05% carbon, 0.99% manganese, 0.75% silicon, and the balance iron. By heat-treating the shot in vacuum, it is possible to manufacture a peening shot in which the material of the central portion 1 is SUS310S and the surface layer is the layer 2 containing a large amount of chromium.

FIGS. 1B and 1C show an enlarged view of the main part in the vicinity of the surface obtained by enlarging part A in FIG. 1A, and the relationship between the amount of chromium and the distance from the surface in line B in FIG. FIG.

In the layer 2 containing a large amount of chromium, the amount of chromium is higher than that in the center in the range from the surface to 5 μm. The amount of chromium in the surface portion is as large as 50%, and as the distance from the surface increases, the amount of chromium approaches approximately 25% of SUS310S as a raw material. By using a shot containing a large amount of chromium on the surface shown in FIG. 1 for peening, a compressive residual stress layer can be formed on the surface of the material, and a coating layer of a high corrosion resistance element can be formed by transferring the shot surface layer. It becomes possible. At this time, in order to form the coating layer of the high corrosion resistance element, the thickness of the surface layer containing a large amount of chromium needs to be at least 1 μm or more.

In this embodiment, the amount of chromium is 50%. However, the hardness of the peening effect is sufficient if the amount of chromium is large. If it is large, the amount of chromium on the surface to be processed can be reduced in a shorter shot processing time. Can do a lot. For this reason, it is sufficient that the chromium concentration is higher than the chromium concentration on the surface of the workpiece.

  FIG. 2 is a flowchart of a method for manufacturing a peening shot according to the present invention. First, a spherical shot is created from an iron-chromium-nickel alloy material (S1) (S2). For example, a round shot with a diameter of 3 mm can be created by cutting a round bar (φ3 mm) made of SUS310S into a length of 3 mm and processing it into a spherical shape.

Next, the shot is heat-treated in vacuum (S3), whereby a layer containing a large amount of chromium can be formed on the shot surface. As a method of holding the shot in vacuum, there are a method using a heating furnace having a vacuum chamber and a method of vacuum-sealing the shot in a quartz tube. At this time, the degree of vacuum is 10 ⁻⁴ Pa or less, the temperature of the heat treatment is 980 to 1020 ° C., and the heat treatment time is 60 to 80 minutes, which is a suitable condition for forming a layer containing a large amount of chromium.

  FIG. 3 is an explanatory view schematically showing the principle of forming a layer containing a large amount of chromium on the shot surface. 3 (b) and 3 (c) are an enlarged view of the main part in the vicinity of the surface in which part A in FIG. 3 (a) is enlarged, and the amount of chromium in the D line in FIG. 3 (b) and the distance from the surface. It is the figure which showed the relationship.

The vapor pressure of chromium is higher than that of iron or nickel. For example, at an absolute temperature of 1373 K, the vapor pressures of chromium and iron are 3.3 × 10 ⁻³ Pa and 8.9 × 10 ⁻⁴ Pa, respectively.

When the spherical iron-chromium-nickel alloy 3 shown in FIG. 3 (a) is heated in a vacuum, chromium having a high vapor pressure is selectively evaporated from the alloy in the vicinity of the surface, and as shown in FIG. 3 (b). The chrome moves from the center to the surface.

FIG. 3C shows the chromium content distribution near the surface, the solid line 4 shows the chromium content before heat treatment, and the broken line 5 shows the chromium content distribution after heat treatment. Since chromium moves from the center toward the surface due to the difference in vapor pressure, the amount of chromium in the vicinity of the surface gradually increases. At this time, if the degree of vacuum is 10 ⁻⁴ Pa or less, the temperature of the heat treatment is 980 to 1020 ° C., and the heat treatment time is 60 to 80 minutes, a layer with a large amount of chromium is formed on the shot surface.

  In addition, as a chemical component of the spherical iron-chromium-nickel alloy 3, instead of SUS310S, chromium moves to the surface by heat treatment, and a layer having a large amount of chromium is formed on the surface. SUS304 containing 19 to 22%, or SUS316 containing 16 to 18% chromium and 10 to 14% nickel may be used.

DESCRIPTION OF SYMBOLS 1 ... Center part 2 ... Surface layer containing many chromium 3 ... Spherical iron-chromium-nickel alloy 4 ... Chromium content distribution before heat processing 5 ... Chromium content distribution after heat processing

Claims (8)

  An iron-chromium-nickel alloy peening shot used for a surface modification method in which a compressive residual stress layer is formed on the material surface layer and chromium, which is an element having high corrosion resistance, is provided on the material surface. An iron-chromium-nickel alloy peening shot characterized in that the amount of chromium in the surface layer is larger than that in the center.   The peening shot according to claim 1, wherein the surface layer containing a large amount of chromium has a thickness of 1 µm or more.   The method for producing a peening shot according to claim 1, wherein a surface layer containing a large amount of chromium in the surface layer is formed by heat-treating a spherical iron-chromium-nickel alloy in a vacuum. 4. The method for producing a peening shot according to claim 3, wherein the heat treatment comprises heating a spherical iron-chromium-nickel alloy in a vacuum of 10 <^-4> Pa or less.   The method for producing a peening shot according to claim 3, wherein the heat treatment is performed by heating the spherical iron-chromium-nickel alloy at a temperature of 980 ° C to 1020 ° C for 60 to 80 minutes. .   4. A peening shot according to claim 3, wherein the chemical component of the spherical iron-chromium-nickel alloy comprises 24-26% by mass of chromium and 19-22% by mass of nickel and the balance iron and inevitable components. Manufacturing method.   4. The peening according to claim 3, wherein the chemical component of the spherical iron-chromium-nickel alloy comprises 18 to 20% by mass of chromium and 8 to 10.5% by mass of nickel, the balance being iron and inevitable components. Method for manufacturing shots.   4. The peening shot according to claim 3, wherein the chemical component of the spherical iron-chromium-nickel alloy comprises 16 to 18% by mass of chromium and 10 to 14% by mass of nickel, the balance being iron and inevitable components. Manufacturing method.

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